This is such a great question because it really makes you think about the scale of the universe! Let's take a look.

A good starting point is the teaspoon. There are 768 teaspoons in a gallon. For those of you on the metric system, there are 203.04 teaspoons in a liter or roughly 5 milliliters per teaspoon.

English Units

Let's start with a couple of common volumes -- the cubic inch and the cubic foot. There are 3.33 teaspoons in a cubic inch. In a cubic foot, there are 12 * 12 * 12 = 1,728 cubic inches. So a cubic foot contains 5,750 or so teaspoons, or 7.5 gallons.

A cubic mile is 5,280 * 5,280 * 5280 = 147,197,952,000 cubic feet. So a cubic mile holds 1,103,984,640,000 gallons (or, in scientific notation, 1.1E+12). That's 8.48E+14 teaspoons.

A cubic light year is pretty big. Light moves at 186,000 miles per second. A light year is the distance that light travels in a year. That's 186,000 * 3,600 * 24 * 365.25 = 5.87E+12 miles, or almost 6 trillion miles. A cubic light year is a cube that is 6 trillion miles on each side. That is 2.02E+38 cubic miles. If you multiply that by 8.48E+14, you get 1.7E+53 teaspoons in a cubic light year! That is 2.23E+50 gallons.

Metric Units

Let's start with the cubic meter. A cubic meter holds 1,000 liters, or 203,000 teaspoons.

A cubic kilometer is 1,000 * 1,000 * 1,000 = 1,000,000,000 cubic meters. So a cubic kilometer holds 1,000,000,000,000 liters (or, in scientific notation, 1.0 E+12). That's 2.0E+14 teaspoons.

Light moves at 297,000 kilometers per second. In the course of a year, that's 297,000 * 3,600 * 24 * 365.25 = 9.37E+12 kilometers, or almost 10 trillion kilometers. A cubic light year is a cube that is 10 trillion kilometers on each side. That is 8.23E+38 cubic kilometers. If you multiply that by 2.0E+14, you get 1.7E+53 teaspoons in a cubic light year! That is 8.5E+50 liters.

The logical next question is, "How many cubic light years are there in the entire universe?" That's harder to answer. The Hubble Space Telescope seems to be seeing galaxies that are on the order of a 10 to 15 billion light years away. That might imply that the universe is a cube that is 30 billion light years on each side. So the entire universe might hold 4.6E+84 teaspoons if you could fill the whole thing up.

Conclusion: The universe is huge!

These links will help you learn more: